Subcutaneous P-Wave Centric Insertable Cardiac Monitor For Long Term Electrocardiographic Monitoring

ABSTRACT

A P-wave centric subcutaneous insertable cardiac monitor for use in performing long term electrocardiographic (ECG) monitoring is disclosed. An implantable housing made of a biocompatible material is provided. At least one pair of ECG sensing electrodes is provided on a ventral (or dorsal) surface and on opposite ends of the implantable housing operatively placed to facilitate sensing in closest proximity to the low amplitude, low frequency content cardiac action potentials that are generated during atrial activation. Electronic circuitry is provided within the housing assembly including a low power microcontroller operable to execute under modular micro program control as specified in firmware, an ECG front end circuit interfaced to the microcontroller and configured to capture the cardiac action potentials sensed by the pair of ECG sensing electrodes which are output as ECG signals, and non-volatile memory electrically interfaced with the microcontroller and operable to continuously store samples of the ECG signals.

FIELD

This application relates in general to electrocardiographic monitoringand, in particular, to a subcutaneous P-wave-centric insertable cardiacmonitor (ICM) for long term electrocardiographic monitoring with specialemphasis on reliably visualizing the P-wave.

BACKGROUND

The electrocardiogram (ECG) was invented by a Dutch physiologist, WillemEinthoven, in 1903. Physicians have since used ECGs to diagnose heartproblems and other medical concerns. The medical and engineeringprinciples underlying Einthoven's work are still applicable today, andalthough ECG machines have evolved to a broad array of differentsystems, over the past century, the fundamental role of an ECG machineremains the same: to record from the skin surface transmembrane ioniccurrents that are generated within the heart during cardiac activationand recovery.

Cardiac depolarization, which is the spread of electrical currentthroughout the heart, originates in the sinoatrial (SA) node in theright atrium and spreads leftward towards the left atrium and inferiorlytowards the atrioventricular (AV) node. Thereafter a delay, occasionedby the AV node, allows atrial blood to enter the ventricles, prior tothe continuation of the depolarization current proceeding down theBundle of His and into the right and left bundle branches, thenadvancing to the Purkinje fibers, and finally spreading to activate theright and left ventricular muscle fibers themselves that lead to theheart muscle squeezing the blood supply forward.

During each cardiac cycle, the transmembrane ionic currents create anelectrical field in and around the heart that can be detected by ECGelectrodes either placed on the skin or implanted under the skin of thethorax to record far field electrical signals from the heart. These farfield electrical signals are the captured ECG signals that can bevisually depicted in an ECG trace as the PQRSTU waveforms, each letterof which represents a specific electrical activity in the heart wellknown to cardiologists. Within each cardiac cycle, these waveformsindicate key aspects of cardiac electrical activity. The critical P-wavecomponent of each heartbeat represents atrial electrical activity, theelectrical signal that is essential if one is to understand heart rhythmdisorders. The QRS components represent ventricular electrical activity,equally critical to understanding heart rhythm disorders. The TUcomponents represent ventricular cell voltages that are the result ofresetting cellular currents in preparation for the next cardiac cycle.The TU components are generally of limited value for the purposes ofunderstanding heart rhythm disorders and are rarely addressed in theanalysis of heart rhythm disorders per se. (Note that the signalsinvolved in the resetting of the atria are so minuscule as to not bevisible in an ECG trace or, even in a standard intra-cardiac recording.)

Practically, the QRS components of the ventricle electrical activity areoften termed the “R-wave,” in brief, as a shorthand way of identifyingventricular electrical activity in its entirety. (Henceforth, theshorthand version of “R-wave” will be used to indicate ventricularactivity and “P-wave” will be used to indicate atrial activity.) These“waves” represent the two critical components of arrhythmia monitoringand diagnosis performed every day hundreds of thousands of times acrossthe United States. Without a knowledge of the relationship of these twobasic symbols, heart rhythm disorders cannot be reliably diagnosed.Visualizing both the P-wave and the R-wave allow for the specificidentification of a variety of atrial tachyarrhythmias (also known assupraventricular tachyarrhythmias, or SVTs), ventriculartachyarrhythmias (VTs), and bradycardias related to sinus node andatrioventricular (AV) node dysfunction. These categories are wellunderstood by cardiologists but only accurately diagnosable if theP-wave and the R-wave are visualized and their relationship and behaviorare clear. Visualization of the R-wave is usually readily achievable, asthe R-wave is a high voltage, high frequency signal easily recorded fromthe skin's surface. However, as the ECG bipole spacing and electrodesurface area decreases, even the R-wave can be a challenge to visualize.To make matters of rhythm identification more complicated, surfaceP-waves can be much more difficult to visualize from the surface becauseof their much lower voltage and signal frequency content. P-wavevisualization becomes exacerbated further when the recording bipoleinter-electrode spacing decreases.

Subcutaneous ECG monitors, because of their small size, have greaterproblems of demonstrating a clear and dependable P-wave. The issuesrelated to a tiny atrial voltage are exacerbated by the small size ofinsertable cardiac monitors (ICMs), the signal processing limits imposedupon them by virtue of their reduced electrode size, and restrictedinter-electrode spacing. Conventional subcutaneous ICMs, as well as mostconventional surface ECG monitors, are notorious for poor visualizationof the P-wave, which remains the primary reason that heart rhythmdisorders cannot precisely be identified today from ICMs. Furthermore,even when physiologically present, the P-wave may not actually appear onan ECG because the P-wave's visibility is strongly dependent upon thesignal capturing ability of the ECG recording device's sensingcircuitry. This situation is further influenced by several factors,including electrode configuration, electrode surface areas and shapes,inter-electrode spacing; where the electrodes are placed on or withinthe body relative to the heart's atria. Further, the presence or absenceof ambient noise and the means to limit the ambient noise is a keyaspect of whether the low amplitude atrial signal can be seen.

Conventional ICMs are generally capable of monitoring a patient's heartrhythm for up to three years and are often used after diagnosticmeasures when dermal ECG monitors fail to identify a suspectedarrhythmia. Consequently, when a physician is strongly suspicious of aserious cardiac rhythm disorder that may have caused loss ofconsciousness or stroke, for example, the physician will often proceedto the insertion of an ICM under the skin of the thorax. Althoughtraditionally, the quality of the signal is limited with ICMs withrespect to identifying the P-wave, the duration of monitoring is hopedto compensate for poor P-wave recording. This situation has led to adependence on scrutiny of R-wave behavior, such as RR interval(R-wave-to-R-wave interval) behavior, often used as a surrogate fordiagnosing atrial fibrillation, a potential cause of stroke. To alimited extent, this approach has some degree of value. Nevertheless,better recording of the P-wave would result in a significant diagnosticimprovement, not only in the case of atrial fibrillation, but in a hostof other rhythm disorders that can result in syncope or loss ofconsciousness, like VT or heart block.

The P-wave is the most difficult ECG signal to capture by virtue oforiginating in the low tissue mass atria and having both low voltageamplitude and relatively low frequency content. Notwithstanding thesephysiological constraints, ICMs are popular, albeit limited in theirdiagnostic yield. The few ICMs that are commercially available today,including the Reveal LINQ ICM, manufactured by Medtronic, Inc.,Minneapolis, Minn., the BioMonitor 2 (AF and S versions), manufacturedby Biotronik SE & Co. KG, Berlin, Germany, and the Abbott Confirm RxICM, manufactured by Abbott Laboratories, Chicago, Ill., all areuniformly limited in their abilities to clearly and consistently sense,record, and deliver the P-wave.

Typically, the current realm of ICM devices use a loop recorder wherecumulative ECG data lasting for around an hour is continuallyoverwritten unless an episode of pre-programmed interest occurs or apatient marker is manually triggered. The limited temporal windowafforded by the recordation loop is yet another restriction on theevaluation of the P-wave, and related cardiac morphologies, and furthercompromises diagnostic opportunities.

For instance, Medtronic's Reveal LINQ ICM delivers long-termsubcutaneous ECG monitoring for up to three years, depending onprogramming. The monitor is able to store up to 59 minutes of ECG data,include up to 30 minutes of patient-activated episodes, 27 minutes ofautomatically detected episodes, and two minutes of the longest atrialfibrillation (AF) episode stored since the last interrogation of thedevice. The focus of the device is more directed to recording durationand programming options for recording time and patient interactionsrather than signal fidelity. The Reveal LINQ ICM is intended for generalpurpose ECG monitoring and lacks an engineering focus on P-wavevisualization. Moreover, the device's recording circuitry is intended tosecure the ventricular signal by capturing the R-wave, and is designedto accommodate placement over a broad range of subcutaneous implantationsites, which is usually sufficient if one is focused on the R-wave givenits amplitude and frequency content, but of limited value in capturingthe low-amplitude, low-frequency content P-wave. Finally, electrodespacing, surface areas, and shapes are dictated (and limited) by thephysical size of the monitor's housing which is quite small, anaesthetic choice, but unrealistic with respect to capturing the P-wave.

Similar in design is the titanium housing of Biotronik's BioMonitor 2but with a flexible silicone antenna to mount a distal electrode lead,albeit of a standardized length. This standardized length mollifies, inone parameter only, the concerns of limited inter-electrode spacing andits curbing effect on securing the P-wave. None of the other factorsrelated to P-wave signal revelation are addressed. Therefore the qualityof sensed P-waves reflects a compromise caused by closely-spaced polesthat fail to consistently preserve P-wave fidelity, with the reality ofthe physics imposed problems of signal-to-noise ratio limitationsremaining mostly unaddressed.

Therefore, a need remains for a continuously recording long-term ICMparticularly attuned to capturing low amplitude cardiac action potentialpropagation from the atria, that is, the P-wave, for accurate arrhythmiaevent capture and subsequent diagnosis.

SUMMARY

Long-term electrocardiographic and physiological monitoring over aperiod lasting up to several years in duration can be provided through acontinuously-recording subcutaneous insertable cardiac monitor (ICM).The sensing circuitry and the physical layout of the electrodes arespecifically optimized to capture electrical signals from thepropagation of low amplitude, relatively low frequency content cardiacaction potentials, particularly the P-waves that are generated duringatrial activation. In general, the ICM is intended to be implantedcentrally and positioned axially and slightly to either the left orright of the sternal midline in the parasternal region of the chest.

One embodiment provides a P-wave centric subcutaneous insertable cardiacmonitor for use in performing long term electrocardiographic (ECG)monitoring. An implantable housing made of a biocompatible material thatis suitable for implantation within a living body is provided. At leastone pair of ECG sensing electrodes is provided on a ventral (or dorsal)surface and on opposite ends of the implantable housing operativelyplaced to facilitate sensing in closest proximity to the low amplitude,low frequency content cardiac action potentials that are generatedduring atrial activation. Electronic circuitry is provided within thehousing assembly including a low power microcontroller operable toexecute under modular micro program control as specified in firmware, anECG front end circuit interfaced to the microcontroller and configuredto capture the cardiac action potentials sensed by the pair of ECGsensing electrodes which are output as ECG signals, and non-volatilememory electrically interfaced with the microcontroller and operable tocontinuously store samples of the ECG signals.

Still other embodiments will become readily apparent to those skilled inthe art from the following detailed description, wherein are describedembodiments by way of illustrating the best mode contemplated. As willbe realized, other and different embodiments are possible and theembodiments' several details are capable of modifications in variousobvious respects, all without departing from their spirit and the scope.Accordingly, the drawings and detailed description are to be regarded asillustrative in nature and not as restrictive.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing, by way of example, a subcutaneous P-wavecentric insertable cardiac monitor (ICM) for long termelectrocardiographic monitoring in accordance with one embodiment.

FIGS. 2 and 3 are respectively top and bottom perspective views showingthe ICM of FIG. 1.

FIG. 4 is a bottom perspective view showing the ICM of FIG. 1 inaccordance with a further embodiment.

FIGS. 5 and 6 are respectively top and bottom perspective views showingan ICM in accordance with a still further embodiment.

FIG. 7 is a plan view showing further electrode configurations.

FIG. 8 is a functional block diagram showing the P-wave focusedcomponent architecture of the circuitry of the ICM of FIG. 1.

FIG. 9 is a functional block diagram showing a system for wirelesslyinterfacing with an ICM in accordance with one embodiment.

FIG. 10 is a flow diagram showing an ICM-implemented method formonitoring ECG data.

FIG. 11 is a functional block diagram showing the signal processingfunctionality of the microcontroller.

FIG. 12 is a functional block diagram showing the operations performedby the download station.

DETAILED DESCRIPTION

Long-term electrocardiographic and physiological monitoring over aperiod lasting up to several years in duration can be provided through acontinuously-recording subcutaneous insertable cardiac monitor (ICM).FIG. 1 is a diagram showing, by way of example, a subcutaneous P-wavecentric ICM 12 for long term electrocardiographic monitoring inaccordance with one embodiment. The ICM 12 is implanted in theparasternal region 11 of a patient 10. The sensing circuitry andcomponents, compression algorithms, and the physical layout of theelectrodes are specifically optimized to capture electrical signals fromthe propagation of low amplitude, relatively low frequency contentcardiac action potentials, particularly the P-waves generated duringatrial activation. The position and placement of the ICM 12 coupled toengineering considerations that optimize the ICM's sensing circuitry,discussed infra, aid in demonstrating the P-wave clearly.

Implantation of a P-wave centric ICM 12 in the proper subcutaneous sitefacilitates the recording of high quality ECG data with a gooddelineation of the P-wave. In general, the ICM 12 is intended to beimplanted anteriorly and be positioned axially and slightly to eitherthe right or left of the sternal midline in the parasternal region 11 ofthe chest, or if sufficient subcutaneous fat exists, directly over thesternum. Optimally, the ICM 12 is implanted in a location leftparasternally to bridge the left atrial appendage. However, eitherlocation to the right or left of the sternal midline is acceptable;placement of the device, if possible, should bridge the vertical heightof the heart, which lies underneath the sternum 7, thereby placing theICM 12 in close proximity to the anterior right atrium and the leftatrial appendage that lie immediately beneath.

The ICM 12 is shaped to fit comfortably within the body under the skinand to conform to the contours of the patient's parasternal region 11when implanted immediately to either side of the sternum 7, but could beimplanted in other locations of the body. In most adults, the proximalend 13 of the ICM 12 is generally positioned below the manubrium 8 but,depending upon patient's vertical build, the ICM 12 may actuallystraddle the region over the manubrium 8. The distal end 14 of the ICM12 generally extends towards the xiphoid process 9 and lower sternumbut, depending upon the patient's build, may actually straddle theregion over or under the xiphoid process 9, lower sternum and upperabdomen.

Although internal tissues, body structures, and tissue boundaries canadversely affect the current strength and signal fidelity of all bodysurface potentials, subsurface low amplitude cardiac action potentials,particularly P-wave signals with a normative amplitude of less than 0.25millivolts (mV) and a normative duration of less than 120 milliseconds(ms), are most apt to be negatively impacted by these factors. Theatria, which generate the P wave, are mostly located posteriorly withinthe thoracic cavity (with the exception of the anterior right atrium,right atrial appendage and left atrial appendage). The majority of theleft atrium constitutes the portion of the heart furthest away from thesurface of the skin on the chest and harbors the atrial tissue mostlikely to be the source of serious arrhythmias, like atrialfibrillation. Conversely, the ventricles, which generate largeramplitude signals, are located anteriorly as in the case of the anteriorright ventricle and most of the anterior left ventricle situatedrelatively close to the skin surface of the central and left anteriorchest. These factors, together with larger size and more powerfulimpulse generation from the ventricles, contribute to the relativelylarger amplitudes of ventricular waveforms.

Nevertheless, as explained supra, both the P-wave and the R-wave arerequired for the physician to make a proper rhythm diagnosis from thedozens of arrhythmias that can occur. Yet, the quality of P-waves ismore susceptible to weakening from distance and the intervening tissuesand structures and from signal attenuation and signal processing thanthe high voltage waveforms associated with ventricular activation. Theadded value of avoiding further signal attenuation resulting from dermalimpedance makes a subcutaneous P-wave centric ICM even more likely tomatch, or even outperform dermal ambulatory monitors designed toanalogous engineering considerations and using similar sensing circuitryand components, compression algorithms, and physical layout ofelectrodes, such as described in U.S. Pat. No. 9,545,204, issued Jan.17, 2017 to Bishay et al.; U.S. Pat. No. 9,730,593, issued Aug. 15, 2017to Felix et al.; U.S. Pat. No. 9,700,227, issued Jul. 11, 2017 to Bishayet al.; U.S. Pat. No. 9,717,433, issued Aug. 1, 2017 to Felix et al.;and U.S. Pat. No. 9,615,763, issued Apr. 11, 2017 to Felix et al., thedisclosures of which are incorporated by reference.

The ICM 12 can be implanted in the patient's chest using, for instance,a minimally invasive subcutaneous implantation instrument or othersuitable surgical implement. The ICM 12 is positioned slightly to theright or left of midline, covering the center third of the chest,roughly between the second and sixth ribs, approximately spanningbetween the level of the manubrium 8 and the level of the xiphoidprocess 9 on the inferior border of the sternum 7, depending upon thevertical build of the patient 10.

During monitoring, the amplitude and strength of action potentialssensed by an ECG devices, including dermal ECG monitors and ICMs, can beaffected to varying degrees by cardiac, cellular, extracellular, vectorof current flow, and physical factors, like obesity, dermatitis, lungdisease, large breasts, and high impedance skin, as can occur indark-skinned individuals. Performing ECG sensing subcutaneously in theparasternal region 11 significantly improves the ability of the ICM 12to counter some of the effects of these factors, particularly high skinimpedance and impedance from subcutaneous fat. Thus, the ICM 12 exhibitssuperior performance when compared to conventional dermal ECG monitorsto existing implantable loop recorders, ICMs, and other forms ofimplantable monitoring devices by virtue of its engineering and provenP-wave documentation above the skin, as discussed in W. M. Smith et al.,“Comparison of diagnostic value using a small, single channel, P-wavecentric sternal ECG monitoring patch with a standard 3-lead Holtersystem over 24 hours,” Am. Heart J., March 2017; 185:67-73, thedisclosure of which is incorporated by reference.

Moreover, the sternal midline implantation location in the parasternalregion 11 allows the ICM's electrodes to record an ECG of optimal signalquality from a location immediately above the strongestsignal-generating aspects of the atrial. Signal quality is improvedfurther in part because cardiac action potential propagation travelssimultaneously along a north-to-south and right-to-left vector,beginning high in the right atrium and ultimately ending in theposterior and lateral region of the left ventricle. Cardiacdepolarization originates high in the right atrium in the SA node beforeconcurrently spreading leftward towards the left atrium and inferiorlytowards the atrioventricular (AV) node. On the proximal end 13, the ECGelectrodes of the ICM 12 are subcutaneously positioned with the upper orsuperior pole (ECG electrode) slightly to the right or left of thesternal midline in the region of the manubrium 8 and, on the distal end14, the lower or inferior pole (ECG electrode) is similarly situatedslightly to the right or left of the sternal midline in the region ofthe xiphoid process 9 and lower sternum 7. The ECG electrodes of the ICM12 are placed primarily in a north-to-south orientation along thesternum 7 that corresponds to the north-to-south waveform vectorexhibited during atrial activation. This orientation corresponds to theaVF lead used in a conventional 12-lead ECG that is used to sensepositive or upright P-waves. In addition, the electrode spacing and theelectrodes' shapes and surface areas mimic the electrodes used in theICM's dermal cousin, designed as part of the optimal P-wave sensingelectrode configuration, such as provided with the dermal ambulatorymonitors cited supra.

Despite the challenges faced in capturing low amplitude cardiac actionpotentials, the ICM 12 is able to operate effectively using only twoelectrodes that are strategically sized and placed in locations ideallysuited to high fidelity P-wave signal acquisition. This approach hasbeen shown to clinically outperform more typical multi-lead monitorsbecause of the improved P-wave clarity, as discussed in W. M. Smith etal., cited supra. FIGS. 2 and 3 are respectively top and bottomperspective views showing the ICM 12 of FIG. 1. Physically, the ICM 12is constructed with a hermetically sealed implantable housing 15 with atleast one ECG electrode forming a superior pole on the proximal end 13and at least one ECG electrode forming an inferior pole on the distalend 14.

When implanted, the housing 15 is oriented most cephalad. The housing 15is constructed of titanium, stainless steel or other biocompatiblematerial. The housing 15 contains the sensing, recordation andinterfacing circuitry of the ICM 12, plus a long life battery. Awireless antenna is integrated into or within the housing 15 and can bepositioned to wrap around the housing's internal periphery or locationsuited to signal reception. Other wireless antenna placement orintegrations are possible.

Physically, the ICM 12 has four ECG electrodes 16, 17, 18, 19. Therecould also be additional ECG electrodes, as discussed infra. The ECGelectrodes include two ventral (or dorsal) ECG electrodes 18, 19 and twowraparound ECG electrodes 16, 17. One ventral ECG electrode 18 is formedon the proximal end 13 and one ventral ECG electrode 19 is formed on thedistal end 14. One wraparound ECG electrode 16 is formedcircumferentially about the proximal end 13 and one wraparound ECGelectrode 17 is formed circumferentially about the distal end 14. Eachwraparound ECG electrode 16, 17 is electrically insulated from itsrespective ventral ECG electrode 18, 19 by a periphery 20, 21.

The four ECG electrodes 16, 17, 18, 19 are programmatically controlledby a microcontroller through onboard firmware programming to enable aphysician to choose from several different electrode configurations thatvary the electrode surface areas, shapes, and inter-electrode spacing.The sensing circuitry can be programmed, either pre-implant or in situ,to use different combinations of the available ECG electrodes (andthereby changing electrode surface areas, shapes, and inter-electrodespacing), including pairing the two ventral ECG electrodes 16, 17, thetwo wraparound ECG electrodes 18, 19, or one ventral ECG electrode 16,17 with one wraparound ECG electrode 18, 19 located on the opposite endof the housing 15. In addition, the periphery 20, 21 can beprogrammatically controlled to logically combine the wraparound ECGelectrode 16, 17 on one end of the ICM 12 with its corresponding ventralECG electrode 18, 19 to form a single virtual ECG electrode with largersurface area and shape. (Although electronically possible, the two ECGelectrodes that are only on one end of the ICM 12, for instance,wraparound ECG electrode 16 and ventral ECG electrode 18, could bepaired; however, the minimal inter-electrode spacing would likely yielda signal of poor fidelity in most situations.)

In a further embodiment, the housing 15 and contained circuitry can beprovided as a standalone ICM core assembly to which a pair of compatibleECG electrodes can be operatively coupled to form a full implantable ICMdevice.

Other ECG electrode configurations are possible. For instance,additional ECG electrodes can be provided to increase the number ofpossible electrode configurations, all of which are to ensure betterP-wave resolution. FIG. 4 is a bottom perspective view showing the ICM12 of FIG. 1 in accordance with a further embodiment. An additional pairof ventral ECG electrodes 22, 23 are included on the housing's ventralsurface. These ventral ECG electrodes 22, 23 are spaced closer togetherthan the ventral ECG electrodes 18, 19 on the ends of the housing 15 anda physician can thus choose to pair the two inner ventral ECG electrodes22, 23 by themselves to allow for minimal electrode-to-electrodespacing, or with the other ECG electrodes 16, 17, 18, 19 to varyelectrode surface areas, shapes, and inter-electrode spacing evenfurther to explore optimal configurations to acquire the P-wave.

Other housing configurations of the ICM are possible. For instance, thehousing of the ICM can be structured to enhance long term comfort andfitment, and to accommodate a larger long life battery or more circuitryor features, including physiologic sensors, to provide additionalfunctionality. FIGS. 5 and 6 are respectively top and bottom perspectiveviews showing an ICM 30 in accordance with a still further embodiment.The ICM 30 has a housing 31 with a tapered extension 32 that isterminated on the distal end with an electrode 34. On a proximal end,the housing 31 includes a pair of ECG electrodes electrically insulatedby a periphery 37 that include a ventral ECG electrode 33 and awraparound ECG electrode 34. In addition, a ventral ECG electrode 36 isoriented on the housing's distal end before the tapered extension 32.Still other housing structures and electrode configurations arepossible.

In general, the basic electrode layout is sufficient to sense cardiacaction potentials in a wide range of patients. Differences in thoracictissue density and skeletal structure from patient to patient, though,can affect the ability of the sensing electrodes to efficaciouslycapture action potential signals, yet the degree to which signalacquisition is affected may not be apparent until after an ICM has beenimplanted and deployed, when the impacts of the patient's physicalconstitution and his patterns of mobility and physical movement on ICMmonitoring can be fully assessed.

In further embodiments, the electrodes can be configured post-implant toallow the ICM to better adapt to a particular patient's physiology. Forinstance, electrode configurations having more than two sensingelectrodes are possible. FIG. 7 is a plan view showing further electrodeconfigurations. Referring first to FIG. 7(a), a single disc ECGelectrode 40 could be bifurcated to form a pair of half-circle ECGelectrodes 41, 42 that could be programmatically selected or combined toaccommodate a particular patients ECG signal characteristics post-ICMimplant. Referring next to FIG. 7(b), a single disc ECG electrode 45could be divided into three sections, a pair of crescent-shaped ECGelectrodes 46, 47 surrounding a central semicircular ECG electrode 48that could similarly be programmatically selected or combined. Stillother ECG electrode configurations are possible.

ECG monitoring and other functions performed by the ICM 12 are providedthrough a micro controlled architecture. FIG. 8 is a functional blockdiagram showing the P-wave focused component architecture of thecircuitry 80 of the ICM 12 of FIG. 1. The circuitry 80 is poweredthrough the long life battery 21 provided in the housing 15. Operationof the circuitry 80 of the ICM 12 is managed by a microcontroller 81,such as the EFM32 Tiny Gecko 32-bit microcontroller, manufactured bySilicon Laboratories Inc., Austin, Tex. The microcontroller 81 hasflexible energy management modes and includes a direct memory accesscontroller and built-in analog-to-digital and digital-to-analogconverters (ADC and DAC, respectively). The microcontroller 81 alsoincludes a program memory unit containing internal flash memory (notshown) that is readable, writeable, and externally programmable.

The microcontroller 81 operates under modular micro program control asspecified in firmware stored in the internal flash memory. Thefunctionality and firmware modules relating to signal processing by themicrocontroller 81 are further described infra with reference to FIG.11. The microcontroller 81 draws power from the battery provided in thehousing 15 and connects to the ECG front end circuit 83. In a furtherembodiment, the front end circuit 83 measures raw dermal electricalsignals using a driven reference signal that eliminates common modenoise, as further described infra.

The circuitry 80 of the ICM 12 also includes a flash memory 82 externalto the microcontroller 81, which the microcontroller 81 uses forcontinuously storing samples of ECG monitoring signal data and otherphysiology, such as respiratory rate, blood oxygen saturation level(SpO₂), blood pressure, temperature sensor, and physical activity, anddevice and related information. The flash memory 82 also draws powerfrom the battery provided in the housing 15. Data is stored in a serialflash memory circuit, which supports read, erase and program operationsover a communications bus. The flash memory 82 enables themicrocontroller 81 to store digitized ECG data. The communications busfurther enables the flash memory 82 to be directly accessed wirelesslythrough a transceiver 85 coupled to an antenna 17 built into (orprovided with) the housing 15, as further described infra with referenceto FIG. 9. The transceiver 85 can be used for wirelessly interfacingover Bluetooth or other types of wireless technologies for exchangingdata over a short distance with a paired mobile device, includingsmartphones and smart watches, that are designed to communicate over apublic communications infrastructure, such as a cellular communicationsnetwork, and, in a further embodiment, other wearable (or implantable)physiology monitors, such as activity trackers worn on the wrist orbody. Other types of device pairings are possible, including with adesktop computer or purpose-built bedside monitor. The transceiver 85can be used to offload stored ECG monitoring data and other physiologydata and information and for device firmware reprogramming. In a furtherembodiment, the flash memory 82 can be accessed through an inductivecoupling (not shown).

The microcontroller 81 includes functionality that enables theacquisition of samples of analog ECG signals, which are converted into adigital representation, as further described infra with reference toFIG. 11. In one mode, the microcontroller 81 implements a loop recorderfeature that will acquire, sample, digitize, signal process, and storedigitized ECG data into available storage locations in the flash memory82 until all memory storage locations are filled, after which existingstored digitized ECG data will either be overwritten through a slidingwindow protocol, albeit at the cost of potentially losing the storeddata that was overwritten, if not previously downloaded, or transmittedwirelessly to an external receiver to unburden the flash memory. Inanother mode, the stored digitized ECG data can be maintainedpermanently until downloaded or erased to restore memory capacity. Datadownload or erasure can also occur before all storage locations arefilled, which would free up memory space sooner, albeit at the cost ofpossibly interrupting monitoring while downloading or erasure isperformed. Still other modes of data storage and capacity recovery arepossible.

The circuitry 80 of the ICM 12 can include functionality toprogrammatically select pairings of sensing electrodes when the ICM 12is furnished with three or more electrodes. In a further embodiment,multiple sensing electrodes could be provided on the ICM 12 to provide aphysician the option of fine-tuning the sensing dipole (or tripole ormultipole) in situ by parking active electrodes and designating anyremaining electrodes inert. The pairing selection can be made remotelythrough an inductive coupling or by the transceiver 85 via, forinstance, a paired mobile device, as further described infra. Thus, thesensing electrode configuration, including number of electrodes,electrode-to-electrode spacing, and electrode size, shape, surface area,and placement, can be modified at any time during the implantation ofthe ICM 12.

In a further embodiment, the circuitry 80 of the ICM 12 can include anactigraphy sensor 84 implemented as a 3-axis accelerometer. Theaccelerometer may be configured to generate interrupt signals to themicrocontroller 81 by independent initial wake up and free fall events,as well as by device position. In addition, the actigraphy provided bythe accelerometer can be used during post-monitoring analysis to correctthe orientation of the ICM 12 if, for instance, the ICM 12 has beeninadvertently implanted upside down, that is, with the ICM's housingoriented caudally, as well as for other event occurrence analyses.

In a still further embodiment, the circuitry 80 of the ICM 12 caninclude one or more physiology sensors. For instance, a physiologysensor can be provided as part of the circuitry 80 of the ICM 12, or canbe provided on the electrode assembly 14 with communication with themicrocontroller 81 provided through a circuit trace. The physiologysensor can include an SpO₂ sensor, blood pressure sensor, temperaturesensor, respiratory rate sensor, glucose sensor, airflow sensor,volumetric pressure sensing, or other types of sensor or telemetricinput sources.

In a yet further embodiment, firmware with programming instructions,including machine learning and other forms of artificialintelligence-originated instructions, can be downloaded into themicrocontroller's internal flash memory. The firmware can includeheuristics to signal patient and physician with alerts over healthconditions or arrhythmias of selected medical concern, such as where aheart pattern particular to the patient is identified and the ICM 12 isthereby reprogrammed to watch for a reoccurrence of that pattern, afterwhich an alert will be generated and sent to the physician (or othercaregiver) through the transceiver 85 via, for instance, a paired mobiledevice. Similarly, the firmware can include heuristics that can bedownloaded to the ICM 12 to actively identify or narrow down a pattern(or even the underlying cause) of sporadic cardiac conditions, forinstance, atrial tachycardia (AT), atrial fibrillation (AF), atrialflutter (AFL), AV node reciprocating tachycardia, ventriculartachycardia (VT), sinus bradycardia, asystole, complete heart block, andother cardiac arrhythmias, again, after which an alert will be generatedand sent to the physician (or other caregiver) through the transceiver85. For instance, an alert that includes a compressed ECG digitizedsample can also be wirelessly transmitted by the ICM 12 upon thetriggering of a preset condition, such as an abnormally low heart ratein excess of 170 beats per minute (bpm), an abnormally low heart ratefalling below 30 bpm, or AF detected by onboard analysis of RR intervalvariability by the microcontroller 61. Finally, a similar methodology ofcreating firmware programming tailored to the monitoring and medicaldiagnostic needs of a specific patient (or patient group or generalpopulation) can be used for other conditions or symptoms, such assyncope, palpitations, dizziness and giddiness, unspecified convulsions,abnormal ECG, transient cerebral ischemic attacks and related syndromes,cerebral infarction, occlusion and stenosis of pre-cerebral and cerebralarteries not resulting in cerebral infarction personal history oftransient ischemic attack, and cerebral infarction without residualdeficits, to trigger an alert and involve the physician or initiateautomated analysis and follow up back at the patient's clinic. Finally,in a still further embodiment, the circuitry 80 of the ICM 12 canaccommodate patient-interfaceable components, including an externaltactile feedback device (not shown) that wirelessly interfaces to theICM 12 through the transceiver 85. A patient 10 can press the externaltactile feedback device to mark events, such as a syncope episode, or toperform other functions. The circuitry 80 can also accommodatetriggering an external buzzer 67, such as a speaker, magnetic resonatoror piezoelectric buzzer, implemented as part of the external tactilefeedback device or as a separate wirelessly-interfaceable component. Thebuzzer 67 can be used by the microcontroller 81 to indirectly outputfeedback to a patient 10, such as a low battery or other error conditionor warning. Still other components, provided as either part of thecircuitry 80 of the ICM 12 or as external wirelessly-interfaceabledevices, are possible.

In a further embodiment, the ECG front end circuit 83 of the ICM 12measures raw dermal electrical signals using a driven reference signal,such as described in U.S. Pat. Nos. 9,700,227, 9,717,433, and 9,615,763,cited supra. The driven reference signal effectively reduces common modenoise, power supply noise and system noise, which is critical topreserving the characteristics of low amplitude cardiac actionpotentials, especially the P wave signals originating from the atria.

The ECG front end circuit 83 is organized into a passive input filterstage, a unity gain voltage follower stage, a passive high passfiltering stage, a voltage amplification and active filtering stage, andan anti-aliasing passive filter stage, plus a reference generator. Thepassive input filter stage passively shifts the frequency response polesdownward to counter the high electrode impedance from the patient on thesignal lead and reference lead, which reduces high frequency noise. Theunity gain voltage follower stage allows the circuit to accommodate avery high input impedance, so as not to disrupt the subcutaneouspotentials or the filtering effect of the previous stage. The passivehigh pass filtering stage includes a high pass filter that removesbaseline wander and any offset generated from the previous stage. Asnecessary, the voltage amplification and active filtering stageamplifies or de-amplifies (or allows to pass-through) the voltage of theinput signal, while applying a low pass filter. The anti-aliasingpassive filter stage 75 provides an anti-aliasing low pass filter. Thereference generator drives a driven reference signal containing powersupply noise and system noise to the reference lead and is connecteddirectly to the patient, thereby avoiding the thermal noise of theprotection resistor that is included as part of the protection circuit72.

When operated standalone, the recording circuitry of the ICM 12 sensesand records the patient's ECG data into an onboard memory. The ICM 12can interoperate with other devices wirelessly through the transceiver85. FIG. 9 is a functional block diagram showing a system 90 forwirelessly interfacing with an ICM 12 in accordance with one embodiment.The ICM 12 is designed for long-term electrocardiographic andphysiological monitoring lasting up to several years in duration. Duringthat time, stored data ECG monitoring data and other physiology andinformation will need to be offloaded and the ICM's firmware may need tobe reprogrammed, and the transceiver 85 enables the ICM 12 tocommunicate with external devices to facilitate these functions.

In one embodiment, the ICM 12 can be wirelessly connected to a downloadstation 92 executing data link software (“DL”) 93 that permits thesecure remote retrieval of stored ECG monitoring data, execution ofdiagnostics on or programming of the ICM 12, or performance of otherfunctions. The ICM 12 connects to the download station 92 over awireless network 91 via the transceiver 85. In turn, the downloadstation 92 executes the data link software 93 or similar program thatwirelessly interacts with the ICM 12 to retrieve the stored ECGmonitoring data or perform other function. The download station 92 couldbe a server, personal computer, tablet or handheld computer, smartmobile device, or purpose-built programmer designed specific to the taskof interfacing with a ICM 12. Still other forms of download station 92are possible.

Upon retrieving stored ECG monitoring data from a ICM 12, middleware(not shown) executing on the download station 92 first operates on theretrieved data to adjust the ECG capture quality, as necessary, and toconvert the retrieved data into a format suitable for use by third partypost-monitoring analysis software, as further described infra withreference to FIG. 12. The formatted data can then be retrieved from thedownload station 92. The middleware could alternatively be executed by aseparate device other than the download station 92.

A client-server model could be used to employ a server 94 to remotelyinterface with the download station 92 over the network 91 and retrievethe formatted data or other information. The server 94 executes apatient management program 95 (“Mgt”) or similar application that storesthe retrieved formatted data, recorded physiology, and other informationin a secure database 96 cataloged in that patient's electronic medicalrecords (EMRs) 97, along with tracking and correlating patient symptoms.In addition, the patient management program 95 could manage asubscription service that authorizes a ICM 12 to operate for a setperiod of time or under pre-defined operational parameters.

The patient management program 95, or other trusted application, alsomaintains and safeguards the secure database 96 to limit access topatient EMRs 97 to only authorized parties for appropriate medical orother uses, such as mandated by state or federal law, such as under theHealth Insurance Portability and Accountability Act (HIPAA) or per theEuropean Union's Data Protection Directive. For example, a physician mayseek to review and evaluate his patient's ECG monitoring data, assecurely stored in the secure database 96.

Physician and other authorized healthcare personnel are able to securelyaccess the retrieved formatted data and other information stored in theEMRs 97 in the secure database 96 by executing an application program(“MD”) 98, such as a post-monitoring ECG analysis program, on a personalcomputer 99 or other connectable computing device, and, through theapplication program 98, coordinate access to his patient's EMRs 97 withthe patient management program 95 and perform other functions. Theapplication program 98 can include the capability to actively orinteractively diagnose or narrow down the underlying cause of sporadiccardiac conditions, for instance, atrial tachycardia (AT), AF, atrialflutter, AV node reciprocating tachycardia, ventricular tachycardia(VT), sinus bradycardia, asystole, complete heart block, and othercardiac arrhythmias. Other diagnoses are possible.

In a further embodiment, RR interval data can be extracted from theretrieved formatted data and be presented to physicians in a format thatincludes views of relevant near field and far field ECG data, whichtogether provide contextual information that improves diagnosticaccuracy, such as described in U.S. Pat. No. 9,408,551, issued Aug. 9,2016 to Bardy et al., the disclosure of which is incorporated byreference. Both near field and far field ECG data views are temporallykeyed to an extended duration RR interval data view. The durations ofthe classical “pinpoint” view, the pre- and post-event “intermediate”view, and the RR interval plot are flexible and adjustable. Thus, thepinpoint “snapshot” and intermediate views of ECG data with the extendedterm RR interval data allow a physician to comparatively view heart ratecontext and patterns of behavior prior to and after a clinicallymeaningful arrhythmia, patient concern or other indicia, therebyenhancing diagnostic specificity of cardiac rhythm disorders andproviding physiological context to improve diagnostic ability.

As a result, with the assistance of the server 94, a complete end-to-endclosed loop of patient care can be provided, with the ICM 12 providinglong-term ECG and physiology monitoring and data reporting, the patientmanagement program 95 managing ECG and physiology data retrieval andpatient symptom tracking and correlation, the application program 98empowering physicians with the ability to effectively identify theunderlying cause of sporadic cardiac conditions, particularly cardiacrhythm disorders, and the ICM 12 again facilitating patient followingupon diagnosis and throughout treatment.

In a further embodiment, the ICM 12 can interoperate wirelessly withother physiology monitors and activity sensors 104, whether implanted ordermal, such as activity trackers worn on the wrist or body, and withmobile devices 102, including smartphones and smart watches, that aredesigned to communicate over a public communications infrastructure,such as a cellular communications network. Wearable physiology monitorsand activity sensors 104 encompass a wide range of wirelesslyinterconnectable devices that measure or monitor a patient'sphysiological data, such as heart rate, temperature, blood pressure,respiratory rate, blood pressure, blood sugar (with appropriatesubcutaneous probe), oxygen saturation, minute ventilation, and so on;physical states, such as movement, sleep, footsteps, and the like; andperformance, including calories burned or estimated blood glucose level.

The physiology sensors in non-wearable mobile devices 102, particularlysmartphones, are generally not meant for continuous tracking and do notprovide medically precise and actionable data sufficient for a physicianto prescribe a surgical, catheter or serious drug intervention; suchdata can be considered screening information that something may bewrong, but not data that provides the highly precise information thatmay allow for a surgery, such as implantation of a pacemaker for heartblock or a defibrillator for ventricular tachycardia, or the applicationof serious medications, like blood thinners for atrial fibrillation or acardiac ablation procedure. Such devices, like smartphones, are bettersuited to pre- and post-exercise monitoring or as devices that canprovide a signal that something is wrong, but not in the sufficientdetail and FDA approved, legally meaningful validation to allow formedical action. Conversely, medically actionable wearable sensors anddevices sometimes provide continuous recording for relatively short timeperiods, up to 80 days, but do not span years and, further, must bepaired with a smartphone or computer to offload and evaluate therecorded data, especially if the data is of urgent concern.

Wearable physiology monitors and activity sensors 104, also known as“activity monitors,” and to a lesser extent, “fitness” sensor-equippedmobile devices 102, can trace their life-tracking origins to ambulatorydevices used within the medical community to sense and recordtraditional medical physiology that could be useful to a physician inarriving at a patient diagnosis or clinical trajectory, as well as fromoutside the medical community, from, for instance, sports or lifestyleproduct companies who seek to educate and assist individuals withself-quantifying interests. Data is typically tracked by the wearablephysiology monitors or activity sensors 104 and mobile device 102 foronly the personal use of the wearer. The physiological monitoring isstrictly informational, even where a device originated within themedical community, and the data is generally not time-correlated tophysician-supervised monitoring. Importantly, medically-significantevents, such as cardiac rhythm disorders, including tachyarrhythmias,like ventricular tachycardia or atrial fibrillation, andbradyarrhythmias, like heart block, while potentially detectable withthe appropriate diagnostic heuristics, are neither identified nor actedupon by the wearable physiology monitors and activity sensors 104 andthe mobile device 102.

Frequently, wearable physiology monitors and activity sensors 104 arecapable of wirelessly interfacing with mobile devices 102, particularlysmart mobile devices, including so-called “smartphones” and “smartwatches,” as well as with personal computers and tablet or handheldcomputers, to download monitoring data either in real-time or inbatches. The wireless interfacing of such activity monitors is generallyachieved using transceivers that provide low-power, short-range wirelesscommunications, such as Bluetooth, although some wearable physiologymonitors and activity sensors 104, like their mobile device cohorts,have transceivers that provide true wireless communications services,including 4G or better mobile telecommunications, over atelecommunications network. Other types of wireless and wiredinterfacing are possible.

In a further embodiment, where the wearable physiology monitors andactivity sensors 104 are paired with a mobile device 102, the mobiledevice 102 executes an application (“App”) 103 that can retrieve thedata collected by the wearable physiology monitor and activity sensor104 and evaluate the data to generate information of interest to thewearer, such as an estimation of the effectiveness of the wearer'sexercise efforts. Where the wearable physiology monitors and activitysensors 104 has sufficient onboard computational resources, the activitymonitor itself executes an app without the need to relay data to amobile device 102. The app can include or be supplemented bydownloadable programming instructions, including machine learning andother forms of artificial intelligence-originated instructions. The appcan include heuristics to signal patient and physician with alerts overhealth conditions or arrhythmias of selected medical concern, such aswhere a heart pattern particular to the patient is identified and themobile device 102, in collaboration with the ICM 12, is therebyreprogrammed to watch for a reoccurrence of that pattern, after which analert will be generated and sent to the physician (or other caregiver).Similarly, the app can include heuristics that can actively identify ornarrow down a pattern (or even the underlying cause) of sporadic cardiacconditions, for instance, atrial tachycardia (AT), atrial fibrillation(AF), atrial flutter (AFL), AV node reciprocating tachycardia,ventricular tachycardia (VT), sinus bradycardia, asystole, completeheart block, and other cardiac arrhythmias, again, after which an alertwill be generated and sent to the physician (or other caregiver). Forinstance, an alert that includes a compressed ECG digitized sample canalso be wirelessly transmitted by the app upon the triggering of apreset condition, such as an abnormally low heart rate in excess of 170beats per minute (bpm), an abnormally low heart rate falling below 30bpm, or AF detected by onboard analysis of RR interval variability bythe app. Finally, a similar methodology of creating app programmingtailored to the monitoring and medical diagnostic needs of a specificpatient (or patient group or general population) can be used for otherconditions or symptoms, such as syncope, palpitations, dizziness andgiddiness, unspecified convulsions, abnormal ECG, transient cerebralischemic attacks and related syndromes, cerebral infarction, occlusionand stenosis of pre-cerebral and cerebral arteries not resulting incerebral infarction personal history of transient ischemic attack, andcerebral infarction without residual deficits, to trigger an alert andinvolve the physician or initiate automated analysis and follow up backat the patient's clinic. Still other activity monitor and mobile devicefunctions on the collected data are possible.

In a yet further embodiment, a wearable physiology monitor, activitysensor 104, or mobile device 102 worn or held by the patient 10, orotherwise be used proximal to the patient's body, can be used to firstobtain and then work collaboratively with the more definitive andcapable ICM 12 to enable the collection of physiology by the ICM 12before, during, and after potentially medically-significant events. Thewearable physiology monitor, activity sensor 104, or mobile device 102must be capable of sensing cardiac activity, particularly heart rate orrhythm, or other types of physiology or measures, either directly orupon review of relayed data. Where the wearable physiology monitor oractivity sensor 104 is paired with a mobile device 102, the mobiledevice 102 serves as a relay device to trigger a medical alert upondetecting potentially medically-significant events in the data providedby the paired activity monitor, such as cardiac rhythm disorders,including tachyarrhythmias and bradyarrhythmias. Finally, if thewearable physiology monitor or activity sensor 104 has sufficientonboard computational resources and also is equipped with a wirelesscommunications services transceiver, the wearable physiology monitor oractivity sensor 104 effectively becomes the mobile device and executesan application (not shown) that will trigger the medical alert directly.Still other configurations of the detection app are possible.

In a still further embodiment, the monitoring data recorded by the ICM12 can be uploaded directly into the patient's EMRs 97, either by usinga mobile device 102 as a conduit for communications with the securedatabase 96 via the server 94, or directly to the server 94, if the ICM12 is appropriately equipped with a wireless transceiver or similarexternal data communications interface. Thus, the data recorded by theICM 12 would directly feed into the patient's EMRs 97, thereby allowingthe data to be made certifiable for immediate use by a physician orauthorized healthcare provider. No intermediate steps would be necessarywhen going from subcutaneously sensing cardiac electric signals andcollecting the patient's physiology using a ICM 12 to presenting thatrecorded data to a physician or healthcare provider for medicaldiagnosis and care. The direct feeding of data from the ICM 12 to theEMRs 97 clearly establishes the relationship of the data, as recorded bythe ICM 12, to the patient 10 that the physician is seeing andappropriately identifies any potentially medically-significant eventrecorded in the data as originating in the patient 10 and nobody else.Based on the monitoring data, physicians and healthcare providers canrely on the data as certifiable and can directly proceed withdetermining the appropriate course of treatment for the patient 10,including undertaking further medical interventions as appropriate.

In a yet further embodiment, the server 94 can evaluate the recordeddata, as fed into the patient's EMRs 97, to refer the patient 10 formedical care to a general practice physician or medical specialist, forinstance, a cardiac electrophysiologist referral from a cardiologistwhen the recorded data indicates an event of sufficient potentialseverity to warrant the possible implantation of a pacemaker for heartblock or a defibrillator for ventricular tachycardia. Other uses of thedata recorded by the ICM 12 are possible. For instance, a patient 10 whohas previously suffered heart failure is particularly susceptible toventricular tachycardia following a period of exercise or strenuousphysical activity. A wearable sensor 104 or device 102 that includes aheart rate monitor would be able to timely detect an irregularity inheart rhythm. The application executed by the sensor 104 or device 102allows those devices to take action by triggering the dispatch of a ICM12 to the patient 10, even though the data recorded by the sensor 104 ordevice 102 is itself generally medically-insufficient for purposes ofdiagnosis and care. Thus, rather than passively recording patient data,the sensor 104 or device 102 takes on an active role in initiating theprovisioning of medical care to the patient 10 and starts a cascade ofappropriate medical interventions under the tutelage of and followed byphysicians and trained healthcare professionals.

In a still further embodiment, based upon machine learning instructionsexecuted by the ICM 12 that generates alerts over health conditions orarrhythmias of selected medical concern, the ICM 12 could upload anevent detection application to the sensor 104 or device 102 to enablethose devices to detect those types of potentially medically-significantevents. Alternatively, the event detection application could bedownloaded to the sensor 104 or device 102 from an online applicationstore or similar online application repository. Finally, the ICM 12could use the sensor 104 or device 102 to generate an appropriate alert,including contacting the patient's physician or healthcare services, viawireless (or wired) communications, upon detecting a potentiallymedically-significant event or in response to a patient prompting.

The mobile device 102 could also serve as a conduit for providing thedata collected by the wearable physiology monitor or activity sensor 104to a server 122, or, similarly, the wearable physiology monitor oractivity sensor 104 could itself directly provide the collected data tothe server 122. The server 122 could then merge the collected data intothe wearer's EMRs 134 in the secure database 124, if appropriate (andpermissible), or the server 122 could perform an analysis of thecollected data, perhaps based by comparison to a population of likewearers of the wearable physiology monitor or activity sensor 104. Stillother server 122 functions on the collected data are possible.

Finally, in a yet further embodiment, the ICM 12 can be interrogatedusing a conventional inductive programmer 100, which could be interfacedto the application program 98 executing on a physician's device, or in astandalone fashion. Inductive interfacing may be necessary where thetransceiver 85 has suffered an error condition or is otherwise unable tocommunicate externally.

The ICM 12 continuously monitors the patient's ECG, heart rate andphysiology over a long period of time lasting up to several years induration. FIG. 10 is a flow diagram showing an ICM-implemented method110 for monitoring ECG data. Initially, upon successful implantation,the microcontroller 61 executes a power up sequence (step 111). Duringthe power up sequence, the voltage of the battery is checked, the stateof the flash memory 62 is confirmed, both in terms of operability checkand available capacity, and microcontroller operation is diagnosticallyconfirmed.

Following satisfactory completion of the power up sequence, an iterativeprocessing loop (steps 112-121) is continually executed by themicrocontroller 61. During each iteration (step 112) of the processingloop, the ECG frontend 63 (shown in FIG. 11) continually senses thedermal ECG electrical signals (step 113, FIG. 10) via the ECG electrodes16 and 17 and is optimized to maintain the integrity of the P-wave. Asample of the ECG signal is read (step 114) by the microcontroller 61 bysampling the analog ECG signal that is output by the ECG front endcircuit 63. Each sampled ECG signal, in quantized and digitized form, isprocessed by signal processing modules as specified in firmware (step115), as described infra, and temporarily staged in a buffer (step 116),pending compression preparatory to storage in the flash memory 62 (step117). Following compression, the compressed ECG digitized sample isagain buffered (step 118), then written to the flash memory 62 (step119) using the communications bus. In a further embodiment, an alertthat includes the compressed ECG digitized sample can also be wirelesslytransmitted upon the triggering of a preset condition (step 120), suchas an abnormally low heart rate in excess of 170 beats per minute (bpm),an abnormally low heart rate falling below 30 bpm, or AF detected byonboard analysis of RR interval variability by the microcontroller 61.Processing continues for an indefinite duration (step 121). Still otheroperations and steps are possible.

The microcontroller 61 operates under modular micro program control thatincludes processing of raw analog ECG signals. FIG. 11 is a functionalblock diagram showing the signal processing functionality 130 of themicrocontroller 61. The microcontroller 61 operates under modular microprogram control as specified in firmware 132. The firmware modules 132include high and low pass filtering 133, and compression 134. Othermodules are possible. The microcontroller 61 has a built-in ADC,although ADC functionality could also be provided in the firmware 132.

The ECG front end circuit 63 first outputs an analog ECG signal, whichthe ADC 131 acquires, samples and converts into an uncompressed digitalrepresentation. The microcontroller 61 includes one or more firmwaremodules 133 that perform filtering. In one embodiment, three low passfilters and two high pass filters are used. Following filtering, thedigital representation of the cardiac activation wave front amplitudesare compressed by a compression module 134 before being written out tostorage 135.

The download station 92 (shown in FIG. 9) executes a data link program(“DL”) 93 or similar program that wirelessly interfaces with the ILR 12to retrieve the stored ECG monitoring data and perform other functions.FIG. 12 is a functional block diagram showing the operations 140performed by the download station 141. The download station 141 could bea server, personal computer (as shown), tablet or handheld computer,smart mobile device, or purpose-built programmer designed specific tothe task of wirelessly interfacing with a ICM 12. Still other forms ofdownload station are possible, including download stations connectedthrough indirect wireless interfacing using, for instance, a smart phoneconnected to the ICM 12 through Bluetooth or Wi-Fi, or over an inductivecoupling.

The download station 141 is responsible for offloading stored ECGmonitoring data from a ICM 12. The download station 141 operates underprogrammable control as specified in software. The stored ECG monitoringdata remotely retrieved from storage 142 on a ICM 12 is firstdecompressed by a decompression module 143, which converts the storedECG monitoring data back into an uncompressed digital representationmore suited to signal processing than a compressed signal. The retrievedECG monitoring data may be stored into local storage (not shown) forarchival purposes, either in original compressed form, or asuncompressed.

The download station 141 can include an array of filtering modules. Forinstance, a set of phase distortion filtering tools 144 may be provided,where corresponding software filters can be provided for each filterimplemented in the firmware executed by the microcontroller 61. Thedigital signals are run through the software filters in a reversedirection to remove phase distortion. For instance, a 45 Hertz high passfilter in firmware may have a matching reverse 45 Hertz high pass filterin software. Most of the phase distortion is corrected, that is,canceled to eliminate noise at the set frequency, but data at otherfrequencies in the waveform remain unaltered. As well, bidirectionalimpulse infinite response (IIR) high pass filters and reverse direction(symmetric) IIR low pass filters can be provided. Data is run throughthese filters first in a forward direction, then in a reverse direction,which generates a square of the response and cancels out any phasedistortion. This type of signal processing is particularly helpful withimproving the display of the ST-segment by removing low frequency noise.

An automatic gain control (AGC) module 145 can also be provided toadjust the digital signals to a usable level based on peak or averagesignal level or other metric. AGC is particularly critical tosingle-lead ECG monitors, where physical factors, such as the tilt ofthe heart, can affect the electrical field generated. On three-leadHolter monitors, the leads are oriented in vertical, horizontal anddiagonal directions. As a result, the horizontal and diagonal leads maybe higher amplitude and ECG interpretation will be based on one or bothof the higher amplitude leads. In contrast, the ICM 12 has only a singlelead that is oriented in the vertical direction, so variations inamplitude will be wider than available with multi-lead monitors, whichhave alternate leads to fall back upon.

In addition, AGC may be necessary to maintain compatibility withexisting ECG interpretation software, which is typically calibrated formulti-lead ECG monitors for viewing signals over a narrow range ofamplitudes. Through the AGC module 145, the gain of signals recorded bythe ICM 12 of the electrocardiography monitor 12 can be attenuated up(or down) to work with FDA-approved commercially available ECGinterpretation.

AGC can be implemented in a fixed fashion that is uniformly applied toall signals in an ECG recording, adjusted as appropriate on arecording-by-recording basis. Typically, a fixed AGC value is calculatedbased on how an ECG recording is received to preserve the amplituderelationship between the signals. Alternatively, AGC can be varieddynamically throughout an ECG recording, where signals in differentsegments of an ECG recording are amplified up (or down) by differingamounts of gain.

Typically, the ICM 12 will record a high resolution, low frequencysignal for the P-wave segment similar to the ICM's dermal cousin, suchas provided with the dermal ambulatory monitors cited supra. However,for some patients, the result may still be a visually small signal.Although high resolution is present, the unaided eye will normally beunable to discern the P-wave segment. Therefore, gaining the signal iscritical to visually depicting P-wave detail. This technique works mostefficaciously with a raw signal with low noise and high resolution, astypically generated by the ICM 12. Automatic gain control applied to ahigh noise signal will only exacerbate noise content and beself-defeating.

Finally, the download station can include filtering modules specificallyintended to enhance P-wave content. For instance, a P-wave based boostfilter 146, which is a form of a pre-emphasis filter, can be applied tothe signal to restore missing frequency content or to correct phasedistortion. Still other filters and types of signal processing arepossible.

In one embodiment, the ICM 12 can simply be inserted with a smallsurgical incision that is the width of the widest part of the ICM,typically the transverse cross section of the thickest aspect of thehousing 15. Blunt dissection thereafter under local anesthesia can beused to create the subcutaneous space to receive the ICM 12, which wouldgenerally be inserted into the implantation site, proximal (housing) endfirst, followed by the distal (electrode assembly) end. In a furtherembodiment, the ICM 12 can be implanted in the patient's chest using,for instance, a minimally invasive subcutaneous implantation instrument,such as described in U.S. Pat. No. 6,436,068, issued Aug. 20, 2002 toBardy, the disclosure of which is incorporated by reference.

While the invention has been particularly shown and described asreferenced to the embodiments thereof, those skilled in the art willunderstand that the foregoing and other changes in form and detail maybe made therein without departing from the spirit and scope.

1. A P-wave centric subcutaneous insertable cardiac monitor (ICM) foruse in performing long term electrocardiographic (ECG) monitoring,comprising: an implantable housing comprised of a biocompatible materialthat is suitable for implantation within a living body; at least threeECG sensing electrodes provided on a ventral surface and on oppositeends of the implantable housing with two of the ECG sensing electrodesprovided on opposite ends of the implantable housing, each of the ECGsensing electrodes operatively placed to facilitate sensing in closestproximity to the low amplitude, low frequency content cardiac actionpotentials of P-wave signals that are generated during atrialactivation; and electronic circuitry provided within the implantablehousing comprising: a low power microcontroller operable to executeunder modular micro program control as specified in firmware,programmatic selection of at least one pairing of the ECG sensingelectrodes to facilitate the sensing of the cardiac action potentials ofthe P-wave signals based on a patient's physiology, an ECG front endcircuit interfaced to the microcontroller and configured to specificallycapture the cardiac action potentials of the P-wave signals sensed bythe pairing of the ECG sensing electrodes, and non-volatile memoryelectrically interfaced with the microcontroller and operable tocontinuously store samples of the cardiac action potentials of theP-wave signals.
 2. A subcutaneous insertable cardiac monitor inaccordance with claim 1, further comprising: at least one of the ECGsensing electrodes further provided on the ventral surface of theimplantable housing between the ECG sensing electrodes provided on theopposite ends of the implantable housing.
 3. A subcutaneous insertablecardiac monitor in accordance with claim 1, further comprising: at leastone of the ECG sensing electrodes further provided to wraparound an endof the implantable housing circumferentially about the ECG sensingelectrodes, electrodes provided on the opposite ends of the implantablehousing, continue around the sides and ends of the implantable housing,and over the planar surface of the implantable housing.
 4. Asubcutaneous insertable cardiac monitor in accordance with claim 3,further comprising: a periphery interposed between the at least one ECGsensing electrode and the ECG sensing electrode about which the at leastone sensing electrode is circumferentially provided, wherein thefirmware comprises programmatic control over the electrically insulativeproperties of the periphery.
 5. A subcutaneous insertable cardiacmonitor in accordance with claim 1, wherein at least one of the ECGsensing electrodes is dissimilar from the other ECG sensing electrodewith respect to one or more of electrode size, shape, surface area, andplacement.
 6. A subcutaneous insertable cardiac monitor in accordancewith claim 1, wherein the ECG front end circuit further comprises adriven reference operable to reduce common mode, power supply and systemnoise and optimize sensing of the cardiac action potentials of theP-wave signals.
 7. A subcutaneous insertable cardiac monitor inaccordance with claim 1, the electronic circuitry further comprising: atransceiver operable to wirelessly interface to an external devicethrough which to at least one of provide the samples of the cardiacaction potentials of the P-wave signals from the non-volatile memory andreceive modular micro program control into the firmware.
 8. Asubcutaneous insertable cardiac monitor in accordance with claim 7,wherein the firmware comprises instructions operable to program themicrocontroller to perform the steps comprising: monitoring the cardiacaction potentials of the P-wave signals sensed by the ECG sensingelectrodes for arrhythmias of selected medical concern and wirelesslysending an alert via the transceiver upon the occurrence of one of thearrhythmias of selected medical concern; executing heuristics toactively identify or narrow down a pattern or underlying cause ofsporadic cardiac conditions and wirelessly sending an alert via thetransceiver upon the occurrence of one of the pattern and the underlyingcause; executing programming tailored to the monitoring and medicaldiagnostic needs of at least one of a specific patient, patient group orgeneral patient population of conditions or symptoms of medical concernand wirelessly sending an alert via the transceiver upon the occurrenceof one of the conditions or symptoms of medical concern.
 9. Asubcutaneous insertable cardiac monitor in accordance with claim 1,further comprising: a patient-interfaceable component selected from thegroup comprising an external tactile feedback device that wirelesslyinterfaces to the ICM through the transceiver and an external buzzerimplemented as part of the external tactile feedback device or as aseparate wirelessly-interfaceable component.
 10. A subcutaneousinsertable cardiac monitor in accordance with claim 1, the pairing ofthe ECG sensing electrodes further comprising: an electrodeconfiguration defining an electrode-to-electrode spacing of between 20mm to 200 mm.
 11. A subcutaneous insertable cardiac monitor inaccordance with claim 1, the pairing of the ECG sensing electrodesfurther comprising: an electrode shape comprising a round disc from 2 mmto 10 mm in diameter.
 12. A subcutaneous insertable cardiac monitor inaccordance with claim 1, the pairing of the ECG sensing electrodesfurther comprising: an electrode shape selected from the groupcomprising circumferentially-shaped and asymmetrically-shapedelectrodes.
 13. A subcutaneous insertable cardiac monitor in accordancewith claim 1, wherein the implantable housing comprises a hermeticallysealed implantable housing defining a rectangular shape with roundededges.
 14. A subcutaneous insertable cardiac monitor in accordance withclaim 1, wherein the implantable housing assembly comprises ahermetically sealed implantable housing defining a rectangular shapewith rounded edges with a tapered extension that is terminated on thedistal end with one of the ECG sensing electrodes.
 15. A subcutaneousinsertable cardiac monitor in accordance with claim 1, wherein the powersource comprises a long life direct current power cell comprised withinthe implantable housing.
 16. A subcutaneous insertable cardiac monitorin accordance with claim 1, further comprising: a physiology sensorcomprised within the implantable housing and electrically coupled to thecircuitry with the physiological sensor operable to sense physiology ofthe living body; and the non-volatile memory further operable to storesamples of the physiology sensed by the physiology sensor.
 17. Asubcutaneous insertable cardiac monitor in accordance with claim 16,wherein the physiology sensor is selected from the group consisting ofan SpO₂ sensor, a blood pressure sensor, a temperature sensor, arespiratory rate sensor, a glucose sensor, an air flow sensor, and avolumetric pressure sensor.